1. Field of the Invention
The present invention relates to a frequency-division multiplex (FDM) technology widely used in the art of communications and measurements, and more particularly to a variable-bandwidth frequency-division multiplex communication system capable of dividing and multiplexing frequency channels with the frequency band of each frequency channel being variable.
2. Description of the Prior Art
Transmultiplexers (TMUX) are equipment for efficiently branching and combining a number of frequency-division multiplex signals according to batch digital signal processing. The transmultiplexers are widely used in the field of communications for mutual transformation between frequency-division multiplex signals and time-division multiplex (TDM) signals. The fundamental concept of transmultiplexers is proposed in Maurice G. Bellanger and Jacques L. Daguet, "TDM-FDM Transmultiplexer: Digital Polyphase and FFT", IEEE Trans., COM-22, No. 9, September 1974.
Conventional transmultiplexers suffer a problem in that if the frequency interval between FDM channels is expressed by .DELTA.f, then the maximum bandwidth of each FDM channel is limited to .DELTA.f. Multimedia communications which are currently drawing much attention require flexible communication paths which are capable of communications in a variety of bandwidths. The conventional transmultiplexers are not suitable for multimedia communications because of the fixed frequency band for each channel. Consequently, research efforts have been directed to transmultiplexers which are able to perform communications in variable bandwidths.
For example, Japanese laid-open patent publication No. 63-200635 (JP, A, 63-200635) discloses a transmultiplexer of the multiple sampling type. The multiple-sampling-type transmultiplexer uses an interpolating digital subfilter, and generates a channel clock and an interpolation clock whose frequency is m times the frequency of the channel clock. The interpolating digital subfilter carries out a predetermined filtering process based on the timing provided by the channel clock and the interpolation clock, and generates a filter output at the sampling rate of the interpolation clock. The filter output is supplied to a Fourier transform circuit.
FIG. 1 of the accompanying drawings shows the transmultiplexer disclosed in the above JP, A, 63-200635 document. In the transmultiplexer, it is assumed that an intermediate-frequency (IF) signal is supplied as an input thereto, a channel interval is represented by .DELTA.f, and the multiplex level by N. The transmultiplexer comprises a local oscillator 101 for generating a local oscillation signal to convert an intermediate-frequency signal to a baseband signal, a multiplex clock generator 109 for generating a multiplex clock having a frequency of N.DELTA.f, a divide-by-N/m frequency divider 115 for frequency-dividing the multiplex clock by N/m, and a divide-by-m frequency divider 116 for frequency-dividing an interpolation clock by m. A mixer 103 is supplied with the intermediate-frequency signal and the local oscillation signal, and another mixer 104 is supplied with the intermediate-frequency signal and the local oscillation signal which has been delayed in phase by .pi./2 by a .pi./2 phase shifter 102. The mixers 103, 104 have respective output terminals connected respectively through low-pass filters (LPFs) 105, 106 to respective analog-to-digital (A/D) converters 107, 108.
The transmultiplexer also includes a switching circuit 111 for carrying out signal branching and sampling. Specifically, the switching circuit 111 is supplied with output signals from the A/D converters 107, 108 and separates the inputted time sequence of signals into N separate output signals in every N samples. The N output signals from the switching circuit 111 are supplied through respective delay units 112-1.about.112-N to respective interpolating digital subfilters 117-1.about.117-N. The delay units 112-1.about.112-N serve to delay the supplied signals by time lags proportional to the order in which the signals come in, for thereby generating timed baseband signals. The transmultiplexer further has a fast Fourier transform (FFT) circuit 114 for effecting a complex fast Fourier transform of N points on respective output signals from the interpolating digital subfilters 117-1.about.117-N. The FFT circuit 114 produces N complex output signals as respective channel output signals of the transmultiplexer.
FIG. 2A shows an arrangement of an interpolating digital subfilter 117-i where (N/2)+1.ltoreq.i.ltoreq.N, and FIG. 2B of the accompanying drawings shows an arrangement of an interpolating digital subfilter 117-i where 1.ltoreq.i .ltoreq.N/2. According to the arrangement shown in FIG. 2A, the interpolating digital subfilter comprises two digital subfilters 121, 122 connected to an input terminal, a delay circuit 124 for delaying an output signal from the digital subfilter 122, and an adder 126 for adding output signals from the digital subfilter 121 and the delay circuit 124. According to the arrangement shown in FIG. 2B, the interpolating digital subfilter comprises two digital subfilters 121, 123 connected to an input terminal, a delay circuit 125 for delaying an output signal from the digital subfilter 123, and an adder 126 for adding output signals from the digital subfilter 121 and the delay circuit 125.
Another document JP, A, 63-200636 reveals a variable-bandwidth FDM signal branching circuit which employs the above multiple-sampling-type transmultiplexer. FIG. 3 illustrates the revealed variable-bandwidth FDM signal branching circuit. As shown in FIG. 3, the variable-bandwidth FDM signal branching circuit has a switch matrix 222 connected to output terminals of a multiple-sampling transmultiplexer (TMUX) 221 having a structure shown in FIG. 1, and k signal interpolation circuits 223-1.about.223-k connected to output terminals of the switch matrix 222. It is assumed that the variable-bandwidth FDM signal branching circuit outputs (k-5) signals each having a bandwidth of .DELTA.f, a signal having a bandwidth of 2.DELTA.f, and a signal having a bandwidth of 3.DELTA.f. The (k-5) signals each having a bandwidth of .DELTA.f are outputted respectively from the signal interpolation circuits 223-6.about.223-k. The signal having a bandwidth of 2.DELTA.f is produced when output signals from the signal interpolation circuits 223-1, 223-2 are transferred through respective frequency shifters 224-1, 224-2 and added to each other by an adder 225-1, and a sum signal from the adder 225-1 is passed through an analog low-pass filter 226-1. The signal having a bandwidth of 3.DELTA.f is produced when output signals from the signal interpolation circuits 223-3.about.223-5 are transferred through respective frequency shifters 224-3.about.224-5 and added to each other by an adder 225-2, and a sum signal from the adder 225-2 is passed through an analog low-pass filter 226-2.
Since the sampling frequency is 2.DELTA.f and the bandwidth of a signal for each channel is restricted to .DELTA.f, the conventional multiple-sampling-type transmultiplexer needs a wide bandwidth for multiplexing channels, and hence requires a signal conversion to a signal sequence with an increased sampling frequency, i.e., an interpolation process. Accordingly, the conventional multiple-sampling-type transmultiplexer necessarily becomes large in circuit scale because of the need for interpolation circuits. A channel multiplexer which employs the above conventional multiple-sampling-type transmultiplexer is also large in circuit scale as it has signal interpolation circuits and frequency shifters. If only some of the channels are associated with frequency shifters so as to reduce the circuit scale, then the channel multiplexer requires N.times.N full matrix switch elements in order to increase the bandwidth of a channel in any arbitrary frequency position. As a result, the channel multiplexer still remains large in circuit scale.